An internal combustion engine provided with a plurality of cylinders has been known. Generally, in each of the cylinders of the internal combustion engine, an intake valve, an exhaust valve, and a piston are provided. Generally, when the piston is moved down with the intake valve opened and the exhaust valve closed, air enters via the intake valve (intake stroke). When the piston is moved up with the intake valve and the exhaust valve closed, the air is compressed (compression stroke). When an air-fuel mixture is ignited using an ignition plug or the like, the air-fuel mixture is combusted and expanded. As a result, the piston is moved down (combustion stroke or expansion stroke). Thereafter, the exhaust valve is opened and the piston is moved up, thereby exhausting exhaust gas (exhaust stroke). In a general four-stroke internal combustion engine, the above-described four strokes are repeated for every 720° in crank angle.
The combustion state in each of the cylinders can be changed depending on a state of the air and the fuel. For example, when temperature is low, when humidity is high, or the like, the combustion state can be deteriorated. When the combustion state is thus deteriorated, misfire can occur. When the misfire occurs in the four-stroke engine, rotational speed (angular speed) of the crankshaft can be greatly fluctuated in the interval of 720° in crank angle. As a result, vibration can be transferred to the vehicle body. Such vibration makes an occupant of the vehicle feel unpleasant. Hence, the misfire needs to be detected for an appropriate countermeasure when the misfire occurs.
Japanese Patent Laying-Open No. 2001-41097 discloses a malfunction diagnosis device for a hybrid vehicle. The malfunction diagnosis device determines whether or not misfire occurs in an engine, based on torque of an engine output shaft or a state of fluctuation of rotational speed. Further, Japanese Patent Laying-Open No. 2001-41097 discloses to suppress the misfire in the engine by increasing an amount of fuel injection to increase fuel pressure, advancing ignition timing, and the like.